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Messier 73 - M73 - Asterism

M73 is a curious entry in the Messier catalogue. It's a small group of four faint stars shaped like a "Y" in the constellation of Aquarius. This grouping was discovered by Messier himself on October 4, 1780, the night he also catalogued M72. He described the object as a cluster of three or four stars with some nebulosity. Subsequent observations by William and John Herschel revealed no nebulosity. Nevertheless, John Herschel included it in his General Catalogue of clusters, nebulae and galaxies (GC) and John Dreyer later added it to the New General Catalogue (NGC). It's now known that no such nebulosity exists and recent measurements indicate that M73 is not a true open cluster, but an asterism.

M73 is located in southern Aquarius close to the border with Capricornus. It's positioned just over a degree east of 9th magnitudeglobular cluster, M72. Finding M73, and also M72, can be challenging since both Aquarius and Capricornus are relatively faint with few obvious stars.

One possible path to M73 is to first locate theta Capricorni (θ Cap - mag. +4.1) and then image a line proceeding in a northwesterly direction for about 9 degrees to reach Albali (ε Aqr - mag. +3.8). Just over halfway along this line is M72, with M73 located a further 1.3 degrees to the east.

M73 is best seen from tropical and Southern Hemisphere locations during the months of July, August and September.

M73 has an apparent magnitude of +9.0 and is therefore a difficult 7x50 or 10x50 binocular object. Larger 20x80 binoculars fair better, although since the four component stars are between 10th and 12th magnitude, it appears unspectacular. Through 100mm (4-inch) telescopes, the shape of the group is apparent with three of the stars blue-white in colour and the other orange. One member appears notably fainter than the others (component magnitudes are +10.4, +11.3, +11.7 and +12.3). With this grouping it's possible to push up the magnification as high as seeing conditions allow. A 150mm (6-inch) reflector or larger scope will easily show the cluster in full detail. In total, M73 spans 2.8 arc minutes of apparent sky. It's often been claimed that the telescopic view of M73 appears fuzzy, suggesting the same observation that Messier himself made.

Scientifically there hasn't been a great deal of interest in M73. The cluster was previously treated as a sparsely populated open cluster. However, evidence has always been thin on the ground and there has been some debate as to whether it is just a chance star alignment. The current belief is that the cluster is an asterism with the four stars located at different distances and moving in different directions. A rough estimate of the average distance of M73 is 2,500 light-years. However, asterisms are useful to astronomers when investigating how open clusters are pulled apart due to gravitational forces.